The telephone industry provides protection against transient events such as voltage spikes and current surges as appear from time to time on transmission lines. Such transient events may be summarily described as individually unpredictable bursts of electrical or electromagnetic energy. The effects of any such transient event occurrence upon a station set, a transmission line, or a central facility may range from a bit of noise on the communication line, to destruction of physical apparatus.
The advent of electrically fragile semiconductor based telecommunication transmission terminals and telephone exchange switching facilities, has served to amplify the severity of these effects.
In the past, it was typical for a printed circuit board to have one layer of insulating material sandwiched between two layers of conducting material. As the miniaturization of components became more prevalent, the demand for available surface area on printed circuit boards increased. Currently, printed circuit boards may have as many as ten layers of insulating and conducting material sandwiched together. Typically, openings or holes are drilled through these multi-layer printed circuit boards for receiving component leads. As printed circuit board material is removed via a drill bit, a cylindrical wall is formed to define an opening. This wall is plated with a conductive material for making contact with the conductive surface of some of the conductive layers. Since some openings having plated walls must not make contact with some of the conductive layers, it is necessary for selected conductive layers to have a non-conductive region electrically insulating them from the plated opening.
Also, it is necessary to provide inter-layer alignment ensuring that these non-conductive portions are approximately centered over holes so that no electrical contact is made with that specific layer. Typically, manufacturing processes can assure tolerances of approximately 10 thousandths of an inch.
Multi-layer printed circuit boards are particularly vulnerable to malfunction due to transient events such as voltage spikes resulting from lightning making contact with telephone lines. Typically, when an event such as this occurs, the surge of voltage provides a current which follows the telephone line occasionally terminating at the ground layer on the printed circuit board interfacing the line with other circuitry. The magnitude of the voltage associated with lightning may be of the order of 1500 volts. Often current may pass over or through an insulator if the distance it must traverse is very small or if the potential difference on either side of the insulator is large. It is found that if a potential difference of 1500 volts or more exists between two conductors separated by a gap of less than 20 thousandths of an inch, current may pass between the conductors and a carbon trace may be etched into the insulating material separating the two conductors. When a carbon trace is formed the printed circuit board is usually permanently damaged, since it is very difficult to repair these carbon traces.
It has been found that providing a minimum distance of 20 thousandths of an inch between ground and conductors of other layers is sufficient to avoid current arcing as a result of most transient events.
It is not uncommon for manufacturers of printed circuit boards to perform a post production test to determine if layers on the printed circuit board are aligned with respect to each other. Post production tests may be costly and time consuming using current methods. These tests involve applying high voltages to printed circuit boards and determining if arcing occurs. This may have destructive effects and is not a preferred method. Light tables and x-ray machines are commonly used to measure inter layer alignment but these tend to be costly and labour intensive.
It is accordingly an object of the invention to provide a method of testing boards after they are manufactured to ensure that the alignment between respective layers is within acceptable predetermined limits.
It is a further object of the invention to provide a method of testing boards after they are manufactured, which does not have any destructive effects.